III-V compounds such as GaN, AlN, AlGaN, InGaN, InAlGaN, and InGaAlBNPAs have unique physical and electronic properties that make them ideal candidates for a variety of electronic and opto-electronic devices. In particular, these materials exhibit a direct band gap structure, high electric field breakdown, and high thermal conductivity. Additionally, materials such as InxAl1-xGaN can be used to cover a wide range of band gap energies, i.e., from 1.9 eV (where x equals 1) to 6.2 eV (where x equals 0).
Until recently, the primary method used to grow Group III nitride semiconductors was metal organic chemical vapor deposition (MOCVD) although other techniques such as molecular beam epitaxy (MBE) have also been investigated. In the MOCVD technique, III-V compounds are grown from the vapor phase using metal organic gases as sources of the Group III metals. For example, typically trimethylaluminum (TMA) is used as the aluminum source and trimethylgallium (TMG) is used as the gallium source. Ammonia is usually used as the nitrogen source. In order to control the electrical conductivity of the grown material, electrically active impurities are introduced into the reaction chamber during material growth. Undoped III-V compounds normally exhibit n-type conductivity, the value of the n-type conductivity being controlled by the introduction of a silicon impurity in the form of silane gas into the reaction chamber during growth. In order to obtain p-type material using this technique, a magnesium impurity in the form of biscyclopentadienymagnesium is introduced into the reactor chamber during the growth cycle. As Mg doped material grown by MOCVD is highly resistive, a high temperature post-growth anneal in a nitrogen atmosphere is required in order to activate the p-type conductivity.
Although the MOCVD technique has proven adequate for a variety of commercial devices, the process has a number of limitations that constrain its usefulness. First, due to the complexity of the various sources (e.g., trimethylaluminum, trimethylgallium, and biscyclopentiadienylmagnesium), the process can be very expensive and one which requires relatively complex equipment. Second, the MOCVD technique does not provide for a growth rate of greater than a few microns per hour, thus requiring long growth runs. The slow growth rate is especially problematic for device structures that require thick layers such as high voltage rectifier diodes that often have a base region thickness of approximately 30 microns. Third, n-type AlGaN layers grown by MOCVD are insulating if the concentration of AlN is high (>50 mol. %). Accordingly, the concentration of AlN in the III-V compound layers forming the p-n junction is limited. Fourth, in order to grow a high-quality III-V compound material on a substrate, the MOCVD technique typically requires the growth of a low temperature buffer layer in-between the substrate and III-V compound layer. Fifth, generally in order to obtain p-type III-V material using MOCVD techniques, a post-growth annealing step is required.
Hydride vapor phase epitaxy or HVPE is another technique that has been investigated for use in the fabrication of III-V compound materials. This technique offers advantages in growth rate, simplicity and cost as well as the ability to grow a III-V compound layer directly onto a substrate without the inclusion of a low temperature buffer layer. In this technique III-V compounds are epitaxially grown on heated substrates. The metals comprising the III-V layers are transported as gaseous metal halides to the reaction zone of the HVPE reactor. Accordingly, gallium and aluminum metals are used as source materials. Due to the high growth rates associated with this technique (i.e., up to 100 microns per hour), thick III-V compound layers can be grown.
The HVPE method is convenient for mass production of semiconductor devices due to its low cost, flexibility of growth conditions, and good reproducibility. Recently, significant progress has been achieved in HVPE growth of III-V compound semiconductor materials. AlGaN, GaN and AlN layers have been grown as well as a variety of structures using this technique. Since this technique does not require low temperature buffer layers, a variety of novel device structures have been fabricated, such as diodes with n-GaN/p-SiC heterojunctions. Furthermore, p-type layers have recently been produced using HVPE, thus allowing p-n or p-i-n junction devices to be fabricated.
In order to fully utilize HVPE in the development and fabrication of III-V compound semiconductor devices, thin layers must be produced, on the order of a micron or less. Conventional HVPE techniques have been unable, however, to grow such layers. As a result, the potential of the HVPE technique for fabricating devices based on Group III semiconductors has been limited.
Accordingly, what is needed in the art is a method and apparatus for growing submicron Group III nitride compounds using HVPE techniques. The present invention provides such a method and apparatus.